I’d recommend NPOI. NPOI is FREE and works exclusively with .XLS files. It has helped me a lot.
Detail: you don’t need to have Microsoft Office installed on your machine to work with .XLS files if you use NPOI.
Check these blog posts:
Creating Excel spreadsheets .XLS and .XLSX in C#
NPOI with Excel Table and dynamic Chart
[UPDATE]
NPOI 2.0 added support for XLSX and DOCX.
You can read more about it here:
NPOI 2.0 series of posts scheduled